Generally, as excavation tools which excavate a ground, the ground, and/or soil, a so-called diameter-enlarged type excavation tool (for example, refer to Patent Document 1) is provided.
This tool includes a device which rotates on a central axis and a bit head, namely, bit wing, rotatable on a rotational axis eccentric from the central axis.
Also, this tool has a structure in which the bit head protrudes radially outward when the device is rotating in one direction, and the bit head retracts radially inward when the device is rotating in the other direction.
Specifically, an attaching hole which is open into the tip face of the device and extends parallel to the central axis, is bored in a position eccentric from the central axis.
A pin hole; which is open into the outer peripheral surface of the device, and passes through a part of the inner peripheral surface of the attaching hole; is formed in the attaching hole. A locking pin is embedded in the pin hole.
The bit head includes
                a bit excavating portion to which a tip made of a hard material, such as a cemented carbide, is fixed, and        an attaching shaft which is integrally connected to the bit excavating portion and is inserted into the attaching hole.A groove for engaging with the locking pin is formed in the outer peripheral surface of the attaching shaft.        
The attaching shaft of the bit head is inserted into the attaching hole of the device, and also the locking pin is inserted into the pin hole from the outer peripheral surface of the device. Thus, the locking pin and the groove of the attaching shaft are engaged with each other.
Therefore, the bit head is retained at the tip of the rotational axis.
Additionally, the bit head has a structure; in which, when the bottom surface in the groove and the outer peripheral surface of the locking pin slide along each other, this sliding movement can make the bit head rotate on the axis of the attaching hole (and attaching shaft) as its rotational axis.
In such an excavation tool, when excavation is performed, the device is rotated in one direction (forward direction). This rotation generates a force of friction among the device, the bit head, an object to be excavated (a mountain, the ground, etc.) and/or a casing top, namely, casing shoe. Also, the force of friction makes the bit head protrude radially outward, and then an excavated hole can be formed.
After the formation of the excavated hole has been completed, the device is rotated in the other direction (backward direction). This rotation generates a force of friction with the object to be excavated and/or a casing top. Also, the force of friction makes the bit head retract radially inward, and then the excavation tool can be withdrawn through the excavated hole.
If the locking pin comes off during an excavation work, the bit head may detach from the device and may remain inside the excavated hole; and then the excavating work will be suspended or stopped. Or a further excavation work is required to be performed again. Accordingly, it is necessary to provide a retaining means so that the locking pin does not come off from the device. As such a retaining means for the locking pin, for example, the use of an elastic member has been suggested as disclosed in Patent Documents 2 to 4.    [Patent Document 1] Japanese Unexamined Patent Application No. H05-065787    [Patent Document 2] Japanese Unexamined Patent Application No. H06-074222    [Patent Document 3] Japanese Unexamined Patent Application No. H08-295268    [Patent Document 4] Japanese Unexamined Patent Application No. H08-295269